Luminance and color separation

ABSTRACT

A luminance and color separation filter unit ( 300 ) for extracting a luminance signal (Y) and two color signals (U, V) from a composite color television signal (CVBS). The filter unit ( 300 ) comprises: acquisition means ( 302 ) to acquire a first sample of the composite color television signal, corresponding to a first pixel and other samples, corresponding to other pixels in a neighborhood of the first pixel; correlation estimation means ( 304 ) to estimate a first set of correlation values representing correlations between the first sample and the respective other samples; penalty estimation means ( 306 ) to estimate a second set of penalty values representing relations between the first sample and the respective other samples; computing means ( 308 ) to compute a third set of combined values by means of combining respective elements of the first set and the second set; selection means ( 310 ) to select a particular sample of the composite color television signal on basis of the corresponding combined value; and decoding means ( 312 ) to determine a final luminance value and two color values corresponding to the first pixel, on basis of the first sample and the particular sample.

The invention relates to a luminance and color separation filter unitfor extracting a luminance signal and two color signals from a compositecolor television signal, comprising a chrominance signal being modulatedon a sub-carrier which is located in the high-frequency part of thefrequency spectrum of the luminance signal.

The invention further relates to an image processing apparatuscomprising:

receiving means for receiving a composite color television signal,comprising a chrominance signal being modulated on a sub-carrier whichis located in the high-frequency part of the frequency spectrum of aluminance signal; and

a luminance and color separation filter unit for extracting theluminance signal and two color signals from the composite colortelevision signal.

The invention further relates to a method of extracting a luminancesignal and two color signals from a composite color television signal,comprising a chrominance signal being modulated on a sub-carrier whichis located in the high-frequency part of the frequency spectrum of theluminance signal.

The invention further relates to a computer program product to be loadedby a computer arrangement, comprising instructions to extract aluminance signal and two color signals from a composite color televisionsignal, comprising a chrominance signal being modulated on a sub-carrierwhich is located in the high-frequency part of the frequency spectrum ofthe luminance signal, the computer arrangement comprising processingmeans and a memory.

With HDTV sets becoming readily available in many markets, digitaltelevision is rapidly gaining popularity. However, analog television isexpected to remain the most important television standard for theforeseeable future. With the advent of increasingly larger televisionsthat exhibit significantly higher resolutions, a continued qualityimprovement of the decoded analog television signal is desirable.

Many artifacts that continue to exist in analog television are caused bythe imperfect separation of luminance and chrominance in composite colorvideo signals. This separation is required due to the fact that thechrominance component (C) is transmitted by modulating it onto asub-carrier in the high-frequency part of the luminance, i.e. gray-value(Y) spectrum, as illustrated in FIG. 1. As both components share thesame frequency space, their separation at the receiver side can only beimperfect and often results in artifacts known as cross-color andcross-luminance.

A first type of low-cost PAL and NTSC decoders use horizontalband-pass/notch filters for Y/C separation. See pages 428433 in “Videodemystified: a handbook for the digital engineer 3rd edition”, by K.Jack. Eagle Rock: LLH Technical Publishing, 2001. ISBN 1-878707-56-6.Here, the notch filter in the luminance path suppresses most of thechrominance, but attenuates the high-frequency luminance as well.Similarly, the band-pass filter in the chrominance path passes thechrominance, but also passes the high-frequency luminance. Hence, thesedecoders suffer from a loss of horizontal luminance resolution andstrong cross-luminance and cross-color artifacts.

A second type, more advanced decoders aim at an improved Y/C separationby using so called comb-filters. See e.g. the article “Three-dimensionalpre- and post-filtering for PAL TV signals”, by D. Teichner, in IEEETransactions in Consumer Electronics, Vol. 34 (1988), No. 1, pp.205-227. This type of decoders exploit the opposite sub-carrier phase ofcertain vertically or temporally adjacent samples to separate theluminance from the chrominance. The basic principle can be explained bytaking a composite PAL sample, F₁ that is encoded at an arbitrary phaseφ:F ₁ =Y+U sin(φ)+V cos(φ)  (1)and a second sample F₂ encoded at 180°+φ, of which it is assumed that itwas encoded from identical luminance and chrominance values:F ₂ =Y+U sin(φ+180°)+V cos(φ+180°)F ₂ =Y−U sin(φ)−V cos(φ)  (2)Now, the addition of F₁ and F₂ and subsequent division by two results inthe separated luminance Y, whereas the subtraction and subsequentdivision by two yields the modulated chrominance U sin(φ)+V cos(φ). Thismeans that perfect Y/C separation is possible if F₁ and F₂ were indeedencoded from highly correlated YUV values.

Current state-of-the-art comb-filters adaptively combine various spatialand temporal comb-filters by filtering along the direction of thehighest detected correlation. See pages 115-118 in“Video-Signalverarbeitung”, by C. Hentschel. Stuttgart: Teubner, 1998.ISBN 3-519-06250X. (See also FIG. 2). However, particularly invertically detailed and/or moving areas, the available comb-filteringdirections are often too limited due to the required oppositesub-carrier phase. As such, even modern 3D comb-filters suffer fromcross-talk artifacts and loss of resolution.

It is an object of the invention to provide a filter unit of the kinddescribed in the opening paragraph with an improved luminance and colorseparation.

This object of the invention is achieved in that the filter unitcomprises:

acquisition means to acquire a first sample of the composite colortelevision signal, corresponding to a first pixel and other samples ofthe composite color television signal, corresponding to other pixels ina neighborhood of the first pixel;

correlation estimation means to estimate a first set of correlationvalues representing correlations between the first sample and therespective other samples, on basis of an initial separation of anapproximation of the luminance signal from the composite colortelevision signal;

penalty estimation means to estimate a second set of penalty valuesrepresenting relations between the first sample and the respective othersamples;

computing means to compute a third set of combined values by means ofcombining respective elements of the first set of correlation values andthe second set of penalty values;

selection means to select a particular sample of the composite colortelevision signal on basis of the corresponding combined value comparedto further combined values of the third set of combined values; and

decoding means to determine at least one final value of a set of valuescomprising a final luminance value and two color values corresponding tothe first pixel on basis of the first sample and the particular sample.

In prior art filter units, e.g. based on comb-filters, the selecteddecoding option, i.e. the particular sample, is only based on thecorrelation between the first sample and the particular sample. For theY/C separation of the first sample in a standard two sample filter unit,an additional sample, with a predetermined sub-carrier phase differencecompared to the first sample, is required. However the number of samplesfulfilling that condition is relatively limited. Besides that, often,e.g. in the case of much image detail or motion, the actual correlationbetween the first sample and the particular sample is relatively small.

In the filter unit according to the invention, a more general approachis used by applying an extended set of candidate samples, i.e. decodingoptions. The selection of the most appropriate sample, i.e. theparticular sample, is based on the correlation between the two firstsample and the particular sample and based on a corresponding penaltyvalue. Therefore, the first sample and the particular samplecorresponding to a pixel within a predetermined spatial or temporalneighborhood of the pixel, corresponding to the first sample, are usedas input for the two sample filter unit. The underlying principle of thefilter unit according to the invention is that comb-filtering is mostdesirable on samples that exhibit the highest correspondence, regardlessof their exact spatial or temporal direction. That means that there is atrade-off between a strict phase requirement, e.g. 180° difference, andcorrelation. E.g. a particular sample and the first sample might havenon-opposite sub-carries phases, but a difference of sub-carries phasesof e.g. 170°. In that case the particular sample might be chosen becauseof its high correlation value, although the difference of sub-carriesphases is 170°. This approach offers a significant increase in decodingoptions, and thereby promises an increase in decoding quality.

In an embodiment according to the invention, the correlation estimationmeans is arranged to compute a first one of the correlation values bymeans of computing a difference between a first luminance value and asecond luminance value, the first luminance value belonging to the firstpixel and being represented by a first sample of the approximation ofthe luminance signal, the second luminance value belonging to a secondone of the pixels in the neighborhood of the first pixel and beingrepresented by a second sample of the approximation of the luminancesignal. Alternatively, chrominance values are applied to estimate thefirst one of the correlation values. The approximation of the luminancesignal is obtained by means of an initial Y/C separation being performedby an initial separation filter. This initial separation filter might bebased on any known type of Y/C separation filter as discussed above,e.g. a horizontal band-pass/notch filters or a known comb-filter.

In an embodiment according to the invention, the penalty estimationmeans is arranged to compute a first one of the penalty values by meansof computing a distance between the first pixel and a second one of thepixels in the neighborhood of the first pixel. The distance betweenpixels is an appropriate measure to determine the appropriateness of thecorresponding samples to be applied for Y/C separation. The bigger thetemporal or spatial difference the less appropriate the sample.

In an embodiment according to the invention, the penalty estimationmeans is arranged to compute a first one of the penalty values by meansof:

computing a first difference between a first sub-carrier phase of thefirst sample of the composite color television signal, corresponding tothe first pixel and a second sub-carrier phase of a first one of theother samples corresponding to other pixels in the neighborhood of thefirst pixel; and

computing a second difference between the first difference and apredetermined value.

For a two-sample filter unit the predetermined value corresponds to180°. For a three-sample filter unit the predetermined value correspondsto 120°. In the latter case the decoding means are arranged to determinethe final luminance value and the two color values corresponding to thefirst pixel on basis of the first sample, the particular sample and afurther one of the other samples corresponding to other pixels in aneighborhood of the first pixel. The deviation from the optimumsub-carrier phase is a relatively good measure to determine theappropriateness of the corresponding samples to be applied for Y/Cseparation. The computation of the deviation from the optimumsub-carrier phase is straightforward.

In an embodiment according to the invention the other pixels in theneighborhood of the first pixel are located in a window which iscentered around the first pixel and located in a first field to whichthe first pixel belongs. Alternatively, a first portion of the otherpixels in the neighborhood of the first pixel are located in a firstwindow which is centered around the first pixel and located in a firstfield to which the first pixel belongs and a second portion of the otherpixels in the neighborhood of the first pixel are located in a secondwindow which is located in a second field. The second window is centeredaround a central pixel. A first option is that the first pixel and thecentral pixel have mutually equal coordinates. A second option is thatthe first pixel and the central pixel are located along a motiontrajectory. That means that the difference between the coordinates ofthe first pixel and the coordinates of the central pixel are determinedby a motion vector, representing motion between parts of the first andsecond field. An advantage of applying multiple windows corresponding tomultiple fields is that the probability of selecting an appropriateparticular sample is relatively high.

It is a further object of the invention to provide an image processingapparatus of the kind described in the opening paragraph with animproved luminance and color separation.

This object of the invention is achieved in that the filter unitcomprises:

acquisition means to acquire a first sample of the composite colortelevision signal, corresponding to a first pixel and other samples ofthe composite color television signal, corresponding to other pixels ina neighborhood of the first pixel;

correlation estimation means to estimate a first set of correlationvalues representing correlations between the first sample and therespective other samples, on basis of an initial separation of anapproximation of the luminance signal from the composite colortelevision signal;

penalty estimation means to estimate a second set of penalty valuesrepresenting relations between the first sample and the respective othersamples;

computing means to compute a third set of combined values by means ofcombining respective elements of the first set of correlation values andthe second set of penalty values;

selection means to select a particular sample of the composite colortelevision signal on basis of the corresponding combined value comparedto further combined values of the third set of combined values; and

decoding means to determine at least one final value of a set of valuescomprising a final luminance value and two color values corresponding tothe first pixel on basis of the first sample and the particular sample.

Optionally, the image processing apparatus comprises a display devicefor displaying images being represented by the luminance signal and thetwo color signals. The image processing apparatus might be a TV.

It is a further object of the invention to provide a method of the kinddescribed in the opening paragraph resulting in an improved luminanceand color separation.

This object of the invention is achieved in that the method comprises:

acquiring a first sample of the composite color television signal,corresponding to a first pixel and other samples of the composite colortelevision signal, corresponding to other pixels in a neighborhood ofthe first pixel;

estimating a first set of correlation values representing correlationsbetween the first sample and the respective other samples, on basis ofan initial separation of an approximation of the luminance signal fromthe composite color television signal;

estimating a second set of penalty values representing relations betweenthe first sample and the respective other samples;

computing a third set of combined values by means of combiningrespective elements of the first set of correlation values and thesecond set of penalty values;

selecting a particular sample of the composite color television signalon basis of the corresponding combined value compared to furthercombined values of the third set of combined values; and

determining at least one final value of a set of values comprising afinal luminance value and two color values corresponding to the firstpixel on basis of the first sample and the particular sample.

It is a further object of the invention to provide a computer programproduct of the kind described in the opening paragraph resulting in animproved luminance and color separation.

This object of the invention is achieved in that, the computer programproduct, after being loaded, provides said processing means with thecapability to carry out:

acquiring a first sample of the composite color television signal,corresponding to a first pixel and other samples of the composite colortelevision signal, corresponding to other pixels in a neighborhood ofthe first pixel;

estimating a first set of correlation values representing correlationsbetween the first sample and the respective other samples, on basis ofan initial separation of an approximation of the luminance signal fromthe composite color television signal;

estimating a second set of penalty values representing relations betweenthe first sample and the respective other samples;

computing a third set of combined values by means of combiningrespective elements of the first set of correlation values and thesecond set of penalty values;

selecting a particular sample of the composite color television signalon basis of the corresponding combined value compared to furthercombined values of the third set of combined values; and

determining at least one final value of a set of values comprising afinal luminance value and two color values corresponding to the firstpixel on basis of the first sample and the particular sample.

Modifications of the filter unit and variations thereof may correspondto modifications and variations thereof of the method described.

These and other aspects of the filter unit, of the image processingapparatus, of the method and of the computer program product accordingto the invention will become apparent from and will be elucidated withrespect to the implementations and embodiments described hereinafter andwith reference to the accompanying drawings, wherein:

FIG. 1 schematically shows a spectrum of a composite PAL video signal;

FIG. 2 schematically show sub-carrier phases of samples in adjacentvideo lines for successive fields;

FIG. 3 schematically shows an embodiment of a filter unit according tothe invention;

FIG. 4 schematically shows another embodiment of a filter unit accordingto the invention which is based on a three sample decoding scheme;

FIG. 5A schematically shows candidate windows in the next, current andprevious fields at fixed position;

FIG. 5B schematically shows candidate windows in the next, current andprevious fields at motion compensated position;

FIG. 6 schematically shows another embodiment of a filter unit accordingto the invention which is arranged to derive the luminance signal fromdecoded chrominance; and

FIG. 7 schematically shows an image processing apparatus according tothe invention.

Same reference numerals are used to denote similar parts throughout thefigs.

FIG. 1 schematically shows a spectrum of a composite PAL video signal.

In order to comprehend the problems involved in Y/C separation, one hasto understand the standards for the transmission of analog colortelevision signals, such as the PAL, NTSC and SECAM standards describedin ITU-R BT.470. For these standards, the requirement of backwardcompatibility to existing black-and-white televisions dictates that thetransmission of chrominance (C) has to take place within the bandavailable for the gray-scales (Y).

For PAL, the chrominance components U and V are amplitude modulated inquadrature onto a sub-carrier frequency of 4.43 MHz The resultingone-dimensional spectrum of the composite PAL video signal isillustrated in FIG. 1. In addition, the sign of the V-component, theso-called V-switch, is inverted every other line to reduce the influenceof phase errors. More formally, the above is described in Equation 3,where {right arrow over (x)} indicates the pixel position in a givenfield n, F_(sc) the sub-carrier frequency and F the resulting compositePAL signal.F({right arrow over (x)},n)=Y({right arrow over (x)},n)+U({right arrowover (x)},n)sin(2πF _(sc) t)±V({right arrow over (x)},n)cos(2πF _(sc)t)  (3)

For NTSC, the somewhat differently defined chrominance components I andQ are amplitude modulated in quadrature onto a sub-carrier frequency of3.58 MHz. As no alternating sign is applied to either chrominancecomponent, there is an increased sensitivity to phase errors that canresult in an erroneous hue of the decoded picture. The one-dimensionalspectrum is similar to that of PAL, except that now the available videobandwidth is limited to approximately 4.2 MHz. Equation 4 formallydefines NTSC encoding:F({right arrow over (x)},n)=Y({right arrow over (x)},n)+I({right arrowover (x)},n)sin(2πF _(sc) t)+Q({right arrow over (x)},n)cos(2πF _(sc)t)  (4)The remainder of this specification discusses the Y/C separation of PALcomposite color video signals. However, the Y/C separation of NTSCsignals is nearly identical to the described separation of PAL signalswith equal V-switches. First a short description of prior art Y/Cseparation filters is provided.

At the television receiver, the required separation of Y and C can onlybe imperfect as both components share the same frequency space. Theearly decoders for PAL and NTSC composite video signals used two simpleone-dimensional horizontal filters to separate luminance and chrominancefrom the composite signal. These filters are so-called notch andband-pass filters.

In the luminance path, a notch filter suppresses frequencies near thesub-carrier frequency to eliminate horizontal chrominance components.Due to the small stop band of the notch filter, high-frequencychrominance components, as they occur on horizontal colored transitions,will be insufficiently attenuated. This introduces cross-talk fromchrominance to luminance, resulting in the so-called cross-luminanceartifacts. Furthermore, the luminance resolution is significantlyreduced, as the notch filter suppresses any luminance components in thestop-band.

In the chrominance path, a band-pass filter separates the high frequencycomponents from the composite signal. Although the pass-band of theband-pass filter contains mostly chrominance information, high-frequencyluminance is present as well. Again, cross-talk will occur as thehigh-frequency luminance will be decoded as chrominance, resulting inthe so-called cross-color artifacts.

The band-pass and notch filters can achieve perfect Y/C separation ifthe luminance and chrominance values of horizontally adjacent samplesare identical, as here the frequency spectrum consists of a DC luminancecomponent and a chrominance component at the sub-carrier frequency.However, if the correlation along the horizontal axis is insufficient,the frequency spectrum contains high-frequency luminance and/orchrominance components. The horizontal separation is now imperfect andresults in cross-talk artifacts in the decoded signal.

In areas where horizontally adjacent samples are insufficientlycorrelated, additional methods for Y/C separation are desirable. Forthat purpose, so-called comb-filters can be used to separate luminanceand chrominance along the vertical or temporal axis. Their underlyingprinciples are similar to those of the standard decoder, i.e. passingthe desired frequency components and suppressing the undesired frequencycomponents.

However, the luminance and chrominance are now modulated with harmonicsof f_(h), i.e. the line frequency, and f_(v), i.e. the picturefrequency. Along with the chosen sub carrier frequencies of PAL andNTSC, this results in interleaved and non-overlapping luminance andchrominance frequency components in the direction where sufficientcorrelation is present. For example, in non-moving areas of the picture,the samples are highly correlated along the temporal axis, and as such,the luminance and chrominance components are interleaved andnon-overlapping along that axis. A filter with a comb-shaped amplituderesponse in that particular direction can therefore be used to separatethe luminance and chrominance.

A typical comb-filter implementation uses two samples with an oppositerelative phases, i.e. having a phase difference of 180° to separateluminance and chrominance. See Equations 1 and 2.

However, perfect separation is only possible if both composite sampleswere encoded from identical Y, U and V values. Only in this case, thepositions of the luminance and chrominance frequency componentscorrespond to those of the comb-filter. Therefore, sufficientcorrelation is required along the comb-filtering direction in order toprevent decoding errors. This is analogous to the horizontalband-pass/notch filters, where sufficient correlation is required alongthe horizontal axis.

An inherent drawback of the standard comb-filter is the low density ofsamples that both meet the required phase relationship, and arespatially and/or temporally adjacent. Due to this limited set ofsamples, situations will occur where neither of the neighboring samplesexhibit sufficient correlation with respect to the current sample,thereby causing artifacts in the decoded video.

FIG. 2 schematically show sub-carrier phases of samples 202, 204, 208,210, 214 and 216 in adjacent video lines 313, 1, 314, 2, 315 and 3 forsuccessive fields 1A, 1B, 2A, 2B, 3A, 3B and 4A. Here, the arrow equalsthe sub-carrier phase, e.g. pointing up denotes 0° and to the rightdenotes 90°. Besides that, pairs of samples 206, 212 and 218 aredepicted which are used for standard comb-filters:

the pair 206 of samples 202 and 204 correspond to a line comb-filter;

the pair 212 of samples 208 and 210 correspond to a frame comb-filter;and

the pair 218 of samples 214 and 216 correspond to a field comb-filter.

FIG. 3 schematically shows an embodiment of a filter unit 300 accordingto the invention. In particular FIG. 3 schematically shows a PALdecoder. The filter unit 300 is provided with a composite colortelevision signal CVBS, comprising a chrominance signal being modulatedon a sub-carrier which is located in the high-frequency part of thefrequency spectrum of the luminance signal. The output of the filterunit 300 comprises a luminance signal Y, a first color signal U and asecond color signal V. The filter unit 300 comprises:

an acquisition unit 302 to acquire a first sample of the composite colortelevision signal, corresponding to a first pixel and other samples ofthe composite color television signal, corresponding to other pixels ina neighborhood of the first pixel;

a correlation estimation unit 304 to estimate a first set of correlationvalues representing correlations between the first sample and therespective other samples, on basis of an initial separation of anapproximation of the luminance signal from the composite colortelevision signal;

a penalty estimation unit 306 to estimate a second set of penalty valuesrepresenting relations between the first sample and the respective othersamples;

a computing unit 308 to compute a third set of combined values by meansof combining respective elements of the first set of correlation valuesand the second set of penalty values;

a selection unit 310 to select a particular sample of the compositecolor television signal on basis of the corresponding combined valuecompared to further combined values of the third set of combined values;

a decoding unit 312 to determine at least one final value of a set ofvalues comprising a final luminance value and two color valuescorresponding to the first pixel on basis of the first sample and theparticular sample. This decoding unit might be any known type of PALdecoding filter, e.g. based on a comb-filter; and

an initial separation filter 314.

The sample acquisition unit 302, the correlation estimation unit 304,penalty estimation unit 306, the computing unit 308, the selection unit310, the decoding unit 312 and the initial separation filter 314 may beimplemented using one processor. Normally, these functions are performedunder control of a software program product. During execution, normallythe software program product is loaded into a memory, like a RAM, andexecuted from there. The program may be loaded from a background memory,like a ROM, hard disk, or magnetically and/or optical storage, or may beloaded via a network like Internet. Optionally an application specificintegrated circuit provides the disclosed functionality.

Next the working of the filter unit 300 according to the invention willbe explained. An important aspect of the filter unit 300 is theselection of related samples. This selection is based uponcharacteristics of the composite color television signal CVBS. In thiscase the selection is performed on a per-sample basis. That means thatfor every first sample to be decoded, the most suitable additionalsample, i.e. the particular sample is chosen. The most suitable sampleis determined by:

the correlation value, being computed by the correlation estimation unit304, as insufficiently correlated samples yield decoding errors; and

the penalty value, being computed by the penalty estimation unit 306.The penalty is based on a phase measurement and optionally a distancemeasurement. Spatially and/or temporally adjacent samples are generallyexpected to have a higher correlation to the current sample thannon-adjacent samples. As such, larger spatial and/or temporal distancesshould be avoided.

With phase, optionally distance and correlation information available, astraightforward approach is to apply the criteria to spatially and/ortemporally adjacent samples: the so-called candidate set, which isgenerated by means of the acquisition unit 302. The optimum sample orcandidate, being selected by means of the selection unit 310 within thatcandidate set serve as input to the decoding unit 312, thereby decodingthe current CVBS sample.

However, determining the correlation between samples constitutes achicken-or-the-egg problem: in order to decode the color televisionsignal CVBS, one needs to know the correlation between samples, which inturn is only available after decoding. To break this cycle, thefiltering is initialized by an initial separation, which is performed bythe initial separation filter 314 which is based on e.g. a combinationof horizontal band-pass/notch filters. Even though this initialseparation is far from perfect, experimental validation has shown itssuitability for this purpose.

The exact size of the candidate window is determined by the horizontaland vertical boundaries t_(x) and t_(y), as shown in Equation 5. Again,F({right arrow over (x)}, n) is the composite sample at the pixelposition {right arrow over (x)} in a given field n. $\begin{matrix}{{{C\left( {\overset{\rightarrow}{x},{n + m}} \right)} = \left\{ {{{F\text{(}\overset{\rightarrow}{x}} + \begin{pmatrix}i \\j\end{pmatrix}},{n + {m\text{)}}}} \right\}}{{with}\text{:}}} & (5) \\{{i \in \left\{ {{- t_{x}},\ldots\quad,t_{x}} \right\}},{j \in \left\{ {{- t_{y}},\ldots\quad,t_{y}} \right\}}} & (6)\end{matrix}$If only spatial candidates are used, i.e. m=0, this leads to Equation 7,i.e. the complete candidate set CS equals the spatial candidate setC({right arrow over (x)}, n).CS={C({right arrow over (x)},n)}  (7)

However, CS might be composed of spatial as well as temporal candidates.For example, consider the candidate set shown in Equation 8 and FIG. 5A,where candidates originate from candidate windows in the previous,current and next field.CS={C({right arrow over (x)},n+1), C({right arrow over (x)},n), C({rightarrow over (x)},n−1)}  (8)

As opposed to temporal windows centered around the current spatialposition, motion compensation is preferably used to increase thecorrelation of temporal candidates by positioning the candidate windowsalong the motion axis. This is illustrated in Equation 9 and FIG. 5B,where D({right arrow over (x)}, n) describes the displacement of thesample at pixel position {right arrow over (x)} in a filed field n tofield n+1. The displacement from field n to n−1 is assumed to be−D({right arrow over (x)}, n), i.e. linear movement. $\begin{matrix}{{CS} = \begin{Bmatrix}{{C\left( {{\overset{\rightarrow}{x} + {D\left( {\overset{\rightarrow}{x},n} \right)}},{n + 1}} \right)},} \\{{C\left( {\overset{\rightarrow}{x},n} \right)},} \\{C\left( {{\overset{\rightarrow}{x} - {D\left( {\overset{\rightarrow}{x},n} \right)}},{n - 1}} \right)}\end{Bmatrix}} & (9)\end{matrix}$

Due to the increase in latency, comb-filters using a next field can beundesirable. Therefore, various configurations are possible using onlyprevious fields. Three examples are illustrated in Equation 10, whererespectively frame and field, field only and frame only comb-filters arespecified. $\begin{matrix}{{{CS} = \left\{ {{C\left( {\overset{\rightarrow}{x},n} \right)},{C\left( {\overset{\rightarrow}{x},{n - 1}} \right)},{C\left( {\overset{\rightarrow}{x},{n - 2}} \right)}} \right\}}{{CS} = \left\{ {{C\left( {\overset{\rightarrow}{x},n} \right)},{C\left( {\overset{\rightarrow}{x},{n - 1}} \right)}} \right\}}{{CS} = \left\{ {{C\left( {\overset{\rightarrow}{x},n} \right)},{C\left( {\overset{\rightarrow}{x},{n - 2}} \right)}} \right\}}} & (10)\end{matrix}$

The computation of the combined value, based on the correlation andphase is as follows. Given a candidate set CS of CSAFAX candidates, acombined value is assigned to each candidate as a function of both thephase relationship and the correlation to the current sample. This isshown in Equation 11, where the combined value ε₁ is calculated for therespective candidates CS₁ with iε{i, . . . , CSMAX}. The combined valueequals the weighted sum of the correlation value L(CS₁, F₁) and thephase penalty P(CS₁) where α₁ and α₂ correspond to the respectiveweighting factors:ε₁(CS ₁ ,F ₁)=α₁ ·L(CS ₁ ,F ₁)+α₂ ·P(CS ₁)  (11)with:F ₁ =F({right arrow over (x)}, n)  (12)

The correlation value is calculated in a straightforward manner as theabsolute difference of the initially separated luminance values, asshown in Equation 13.L(CS ₁ , F ₁)=|Y _(init)(F ₁)−Y _(init)(CS ₁)|  (13)

The basic idea behind the phase penalty is that the phase differencesthat result in no amplification of correlation noise should yield thelowest penalty. For the two sample comb-filter kernel, the situation issimplified as strict phase requirements exist. In the case of identicalV-switches, a two sample comb-filter requires an opposite relativephase, i.e. a difference of 180°, whereas in case of non-identicalV-switches, comb-filtering using two samples is only possible if thesamples' absolute phases are opposite.

First, the sub-carrier phase of F₁ is specified as ∠(F₁). The normalizedphase difference β_(n) which transforms the relative phase β from [0,2π]is determined according to: $\begin{matrix}{{\beta = {{{\angle\left( {CS}_{i} \right)} - {\angle\left( F_{1} \right)}}}}{\beta_{n} = \left\{ \begin{matrix}{{2\quad\pi} - \beta} & {,{if}} & {\beta_{n} > \pi} \\\beta & \quad & {,{else}}\end{matrix} \right.}} & (14)\end{matrix}$Then the phase penalty for samples with identical V-switches can bedefined: $\begin{matrix}{{P\left( {CS}_{i} \right)} = \left\{ \begin{matrix}0 & {,{if}} & {\beta_{n} = \pi} \\1 & \quad & {,{else}}\end{matrix} \right.} & (15)\end{matrix}$In turn, the phase penalty for samples with non-identical V-switches is:$\begin{matrix}{{P\left( {CS}_{i} \right)} = \left\{ \begin{matrix}0 & {,{if}} & {{\angle\left( {CS}_{i} \right)} = {- {\angle\left( F_{1} \right)}}} \\1 & \quad & {,{else}}\end{matrix} \right.} & (16)\end{matrix}$Having determined the penalties for the CSMAX candidates withincandidate set CS, the optimum candidate, i.e. the one with the lowestcombined value, will be selected as the particular sample F₂. Both F₁and F₂ can now be decoded by the two sample comb-filter kernel, i.e. thedecoding unit 312.

Optionally the filter unit 300 comprises an up-sampling unit 316 whichis designed to achieve an increased density of the sampling grid bymeans of interpolation. Within a certain spatial distance, now k timesas many candidates are available in comparison to the original samplinggrid, with k being the up-sampling factor. Therefore, the amount ofcandidates has increased, whereas a deterioration of correlation due toincreased spatial distance has been avoided.

Besides decoding based on two samples, there are decoding techniquesbased on three samples. A filter unit according to the invention of thislatter type is described in connection with FIG. 4. First the decodingbased on three samples is explained and then the method of selectingthree appropriate samples.

FIG. 4 schematically shows another embodiment of a filter unit accordingto the invention, which is based on a three sample decoding scheme. Thefilter unit 400 is provided with a composite color television signalCVBS, comprising a chrominance signal being modulated on a sub-carrierwhich is located in the high-frequency part of the frequency spectrum ofthe luminance signal. The output of the filter unit 400 comprises aluminance signal Y, a first color signal U and a second color signal V.The filter unit 400 comprises:

a sample acquisition unit 402 which is arranged to acquire a first F₁, asecond F₂ and a third F₃ sample from the received composite colortelevision signal CVBS and to regenerate three signals α, β and γcorresponding to the sub-carrier used for encoding of the video data;

a first processing unit 404 for computing a first intermediate signalY_(n);

a second processing unit 406 for computing a second intermediate signalU_(n);

a third processing unit 408 for computing a third intermediate signalV_(n);

a fourth processing unit 410 for computing a fourth intermediate signalE; and

a division unit 412 for computing the luminance signal Y, the firstcolor signal U and the second color signal V on basis of theintermediate signals Y_(n), U_(n), V_(n) and E.

The filter unit 400 is arranged to compute an output luminance value ofa particular output pixel, a first color value of the particular outputpixel and a second color value of the particular output pixel on basis afirst F₁, a second F₂ and a third F₃ sample derived from the compositecolor television signal CVBS, where the first, the second and the thirdsample have mutually different sub-carrier phases.

A received composite sample, F({right arrow over (x)}, n) introducesthree unknown variables, namely the values of Y, U and V, and one knownvalue, i.e. the locally regenerated sub-carrier phase ωt. Basic algebrashows that, given three linear equations, these three unknown variablescan be solved. This means that three composite samples, encoded from Y,U and V values, can be used to separate the Y, U and V componentsexactly. However, in the situation that the composite samples wereencoded from non-identical Y, U and V values, perfect separation is notpossible and errors in the decoded values will occur.

To discuss the decoding of samples with non-opposite phases in moredetail, two situation with respect to the V-switch of three compositesamples should be considered:

The V-switch of all three samples is identical; or

One of the three samples has an unequal V-switch with respect to theother samples.

Therefore a distinction between the decoding of samples with identicalV-switches, and the decoding of samples with non-identical V-switches ismade. Although the following calculations are applicable to PAL signals,identical principles apply to NTSC as to PAL signals with identicalV-switches. Then, the chrominance components I and Q are used instead ofU and V.

In the case of identical V-switches, consider three composite samplesencoded from the same Y, U and V values as shown in Equation 17. Inorder to obtain three independent equations, the phases were chosen tobe unequal, i.e. α≠β≠γ. Also, the V-switch of all V components is chosento be positive. In the case of all negative V-switches, the situation isidentical expect for an inversion of the sign of the decoded V component$\begin{matrix}{{F_{1} = {Y + {{U \cdot \sin}\quad(\alpha)} + {{V \cdot \cos}\quad(\alpha)}}}{F_{2} = {Y + {{U \cdot \sin}\quad(\beta)} + {{V \cdot \cos}\quad(\beta)}}}{F_{3} = {Y + {{U \cdot \sin}\quad(\gamma)} + {{V \cdot \cos}\quad(\gamma)}}}} & (17)\end{matrix}$By solving these three linear equations for the Y, U and V components,the expressions in Equations 18 and 19 are obtained. Here, the Y, U andV components are expressed in terms of the three original compositesamples and their corresponding sub-carrier phase. $\begin{matrix}{{Y_{n} = \begin{matrix}{{{{+ F_{1}} \cdot {\sin(\beta)} \cdot \cos}\quad(\gamma)} - {{F_{1} \cdot \sin}\quad{(\gamma) \cdot \cos}\quad(\beta)}} \\{{{{+ F_{2}} \cdot {\sin(\gamma)} \cdot \cos}\quad(\alpha)} - {{F_{2} \cdot \sin}\quad{(\alpha) \cdot \cos}\quad(\gamma)}} \\{{{{+ F_{3}} \cdot {\sin(\alpha)} \cdot \cos}\quad(\beta)} - {{F_{3} \cdot \sin}\quad{(\beta) \cdot \cos}\quad(\alpha)}}\end{matrix}}{U_{n} = \begin{matrix}{{{+ F_{1}} \cdot {\cos(\beta)}} - {{F_{1} \cdot \cos}\quad(\gamma)} + {{F_{2} \cdot \cos}\quad(\gamma)}} \\{{{- F_{2}} \cdot {\cos(\alpha)}} + {{F_{3} \cdot \cos}\quad(\alpha)} - {{F_{3} \cdot \cos}\quad(\beta)}}\end{matrix}}{V_{n} = \begin{matrix}{{{+ F_{1}} \cdot {\sin(\gamma)}} - {{F_{1} \cdot \sin}\quad(\beta)} + {{F_{2} \cdot \sin}\quad(\alpha)}} \\{{{- F_{2}} \cdot {\sin(\gamma)}} + {{F_{3} \cdot \sin}\quad(\beta)} - {{F_{3} \cdot \sin}\quad(\alpha)}}\end{matrix}}{E = \begin{matrix}{{{{+ {\sin(\alpha)}} \cdot \cos}\quad(\beta)} - {\sin\quad{(\alpha) \cdot \cos}\quad(\gamma)}} \\{{{{+ {\sin(\beta)}} \cdot \cos}\quad(\gamma)} - {\sin\quad{(\beta) \cdot \cos}\quad(\alpha)}} \\{{{{+ {\sin(\gamma)}} \cdot \cos}\quad(\alpha)} - {\sin\quad{(\gamma) \cdot \cos}\quad(\beta)}}\end{matrix}}{{with}\text{:}}} & (18) \\{{Y = \frac{Y_{n}}{E}},{U = \frac{U_{n}}{E}},{V = \frac{V_{n}}{E}}} & (19)\end{matrix}$

A similar calculation can be performed for samples with non-identicalV-switches. Two situations can be distinguished:

The V-switch of one composite sample is positive, whereas the remainingsamples have a negative V-switch; or

The V-switch of one composite sample is negative, whereas the remainingsamples have a positive V-switch.The first situation is shown in Equation 20, whereas the secondsituation will not be covered, as it is identical except for aninversion in sign of the decoded V component. $\begin{matrix}{{F_{1} = {Y + {{U \cdot \sin}\quad(\alpha)} + {{V \cdot \cos}\quad(\alpha)}}}{F_{2} = {Y + {{U \cdot \sin}\quad(\beta)} + {{V \cdot \cos}\quad(\beta)}}}{F_{3} = {Y + {{U \cdot \sin}\quad(\gamma)} + {{V \cdot \cos}\quad(\gamma)}}}} & (20)\end{matrix}$By solving these equations for the Y, U and V components, theexpressions depicted in Equations 21 and 22 can be obtained.$\begin{matrix}{{Y_{n} = \begin{matrix}{{{{+ F_{1}} \cdot {\sin(\beta)} \cdot \cos}\quad(\gamma)} - {{F_{1} \cdot \sin}\quad{(\gamma) \cdot \cos}\quad(\beta)}} \\{{{{- F_{2}} \cdot {\sin(\gamma)} \cdot \cos}\quad(\alpha)} - {{F_{2} \cdot \sin}\quad{(\alpha) \cdot \cos}\quad(\gamma)}} \\{{{{+ F_{3}} \cdot {\sin(\alpha)} \cdot \cos}\quad(\beta)} + {{F_{3} \cdot \sin}\quad{(\beta) \cdot \cos}\quad(\alpha)}}\end{matrix}}{U_{n} = \begin{matrix}{{{+ F_{1}} \cdot {\cos(\beta)}} - {{F_{1} \cdot \cos}\quad(\gamma)} + {{F_{2} \cdot \cos}\quad(\gamma)}} \\{{{+ F_{2}} \cdot {\cos(\alpha)}} - {{F_{3} \cdot \cos}\quad(\alpha)} - {{F_{3} \cdot \cos}\quad(\beta)}}\end{matrix}}{V_{n} = \begin{matrix}{{{+ F_{1}} \cdot {\sin(\beta)}} - {{F_{1} \cdot \sin}\quad(\gamma)} + {{F_{2} \cdot \sin}\quad(\gamma)}} \\{{{- F_{2}} \cdot {\sin(\alpha)}} + {{F_{3} \cdot \sin}\quad(\alpha)} - {{F_{3} \cdot \sin}\quad(\beta)}}\end{matrix}}{E = \begin{matrix}{{{{+ {\sin(\alpha)}} \cdot \cos}\quad(\beta)} - {\sin\quad{(\alpha) \cdot \cos}\quad(\gamma)}} \\{{{{+ {\sin(\beta)}} \cdot \cos}\quad(\gamma)} + {\sin\quad{(\beta) \cdot \cos}\quad(\alpha)}} \\{{{{- {\sin(\gamma)}} \cdot \cos}\quad(\alpha)} - {\sin\quad{(\gamma) \cdot \cos}\quad(\beta)}}\end{matrix}}{{With}\text{:}}} & (21) \\{{Y = \frac{Y_{n}}{E}},{U = \frac{U_{n}}{E}},{V = \frac{V_{n}}{E}}} & (22)\end{matrix}$

Next the computation of penalty values in the case of three samples isspecified. In the case of identical V-switches the phase penalty valueis specified by Equation 23: $\begin{matrix}{{P\left( {CS}_{1} \right)} = \left\{ \begin{matrix}0 & {{,{{{if}\quad\beta_{n}} = \pi}}\quad} \\\frac{{3\quad\beta_{n}} - {2\quad\pi}}{\pi} & {\quad{,{{{if}\quad\frac{2\quad\pi}{3}} < \beta_{n} > \pi}}\quad} \\\frac{{2\quad\pi} - {3\quad\beta_{n}}}{\pi} & {\quad{,{{{if}\quad\frac{\pi}{3}} < \beta_{n} \leq \frac{2\quad\pi}{3}}}} \\1 & {{,{else}}\quad}\end{matrix} \right.} & (23)\end{matrix}$

In the case of non-identical V-switches the phase penalty value isspecified in Equation 25 for 0<α≦π/2, whereas the penalty for the otherthree quadrants, i.e. between π/2, π, 3π/2 and 2π can be mapped to thefirst quadrant by means of α_(n): $\begin{matrix}{\alpha_{n} = \left\{ \begin{matrix}{{2\quad\pi} - \alpha} & {\quad{,{{{if}\quad\frac{3\quad\pi}{2}} < \alpha \leq {2\quad\pi}}}} \\{\alpha - \pi} & {,{{{if}\quad\pi} < \alpha \leq \frac{3\quad\pi}{2}}} \\{\pi - \alpha} & {{,{{{if}\quad\frac{\pi}{2}} < \alpha \leq \pi}}\quad} \\\alpha & {{,{else}}\quad}\end{matrix} \right.} & (24) \\{{P\left( {CS}_{1} \right)} = \left\{ {\begin{matrix}0 & {{,{{{if}\quad\beta_{n}} = \pi}}\quad} \\\frac{{3\quad\beta_{n}} - {2\quad\pi}}{\pi} & {\quad{,{{{if}\quad\frac{2\quad\pi}{3}} < \beta_{n} > \pi}}\quad} \\\frac{{2\quad\pi} - {3\quad\beta_{n}}}{\pi} & {\quad{,{{{if}\quad\frac{\pi}{3}} < \beta_{n} \leq \frac{2\quad\pi}{3}}}} \\1 & {{,{else}}\quad}\end{matrix}{with}} \right.} & (25) \\\begin{matrix}{S_{1}\text{:}} & {{{\angle\left( {CS}_{1} \right)} = {- \alpha}}\quad} \\{S_{2}\text{:}} & {\quad{\left( {0 \leq \alpha_{n} < \frac{\pi}{4}} \right)\bigwedge\left( {{2\quad\pi_{n}} < \beta_{n} < \frac{\pi}{2}} \right)}} \\{S_{3}\text{:}} & {\quad{\left( {\frac{\pi}{2} \leq \alpha_{n} < \frac{\pi}{2}} \right)\bigwedge\left( {\frac{\pi}{2} < \beta_{n} < {2\quad\pi_{n}}} \right)}}\end{matrix} & (26)\end{matrix}$

FIG. 6 schematically shows another embodiment of a filter unit accordingto the invention which is arranged to derive the luminance signal fromdecoded chrominance. The filter unit 600 according to the inventioncomprises:

a first low pass filter 602 for filtering a first U one of the two colorsignals;

a second low pass filter 604 for filtering a second V one of the twocolor signals;

a modulator 606 connected to the first low pass filter 602 and thesecond low pass filter 604, for remodulating the filtered first U_(LPF)one of the two color signals and the filtered second V_(LPF) one of thetwo color signals; and

a subtraction unit 608 for subtracting the output of the modulator 606from the composite color television signal CVBS, resulting in aluminance signal Y.

The first 602 and second low pass filter 604 have a characteristic whichmatches the low pass filters being applied in PAL encoders, i.e. 1.3 MHzand the modulator 606 is arranged to modulate with a sub-carrier beingapplied in PAL encoders. In this embodiment according to the inventionthe two filtered color signals U_(LPF) and V_(LPF) do not or hardlycomprise frequency components which were not present in the originalcolor signals before encoding. Furthermore, the luminance signal alsobetter matches the original luminance signal before encoding by a videoencoding unit, i.e. a PAL encoder.

A further improvement of the filter unit according to the invention isbased on dynamic window resizing. Dynamic window resizing can achieve areduction in computational cost and prevent decoding errors due to anerroneous initialization. In flat areas, i.e. in case of highlycorrelated samples, the candidate window size will be decreased to avoidany errors due to inaccuracies in the initialization. In areas with asignificant amount of detail, an enlargement of the window size isnecessary to ensure sufficient correlated candidates are available tothe comb-filter.

FIG. 7 schematically shows an image processing apparatus 700 accordingto the invention, comprising:

Receiving means 302 for receiving a signal representing input images.

The filter unit 706 as described in connection with any of the FIGS. 3,4 and 6; and

A display device 704 for displaying images being represented by theluminance signal and the two color signals.

The signal may be a broadcast signal received via an antenna or cablebut may also be a signal from a storage device like a VCR (VideoCassette Recorder) or Digital Versatile Disk (DVD). The signal isprovided at the input connector 710. The image processing apparatus 700might e.g. be a TV. Alternatively the image processing apparatus 704does not comprise the optional display device but provides the outputimages to an apparatus that does comprise a display device 704. Then theimage processing apparatus 700 might be e.g. a VCR player. Optionallythe image processing apparatus 700 comprises storage means, like ahard-disk or means for storage on removable media, e.g. optical disks.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention and that those skilled in the art willbe able to design alternative embodiments without departing from thescope of the appended claims. In the claims, any reference signs placedbetween parentheses shall not be constructed as limiting the claim. Theword ‘comprising’ does not exclude the presence of elements or steps notlisted in a claim. The word “a” or “an” preceding an element does notexclude the presence of a plurality of such elements. The invention canbe implemented by means of hardware comprising several distinct elementsand by means of a suitable programmed computer. In the unit claimsenumerating several means, several of these means can be embodied by oneand the same item of hardware.

1. A luminance and color separation filter unit (300) for extracting aluminance signal (Y) and two color signals (U, V) from a composite colortelevision signal (CVBS), comprising a chrominance (C) signal beingmodulated on a sub-carrier which is located in the high-frequency partof the frequency spectrum of the luminance signal (Y), the filter unit(300) comprising: acquisition means (302) to acquire a first sample ofthe composite color television signal, corresponding to a first pixeland other samples of the composite color television signal,corresponding to other pixels in a neighborhood of the first pixel;correlation estimation means (304) to estimate a first set ofcorrelation values representing correlations between the first sampleand the respective other samples, on basis of an initial separation ofan approximation of the luminance signal from the composite colortelevision signal; penalty estimation means (306) to estimate a secondset of penalty values representing relations between the first sampleand the respective other samples; computing means (308) to compute athird set of combined values by means of combining respective elementsof the first set of correlation values and the second set of penaltyvalues; selection means (310) to select a particular sample of thecomposite color television signal on basis of the corresponding combinedvalue compared to further combined values of the third set of combinedvalues; and decoding means (312) to determine at least one final valueof a set of values comprising a final luminance value and two colorvalues corresponding to the first pixel, on basis of the first sampleand the particular sample.
 2. A luminance and color separation filterunit (300) as claimed in claim 1, whereby the correlation estimationmeans (304) is arranged to compute a first one of the correlation valuesby means of computing a difference between a first luminance value and asecond luminance value, the first luminance value belonging to the firstpixel and being represented by a first sample of the approximation ofthe luminance signal, the second luminance value belonging to a secondone of the pixels in the neighborhood of the first pixel and beingrepresented by a second sample of the approximation of the luminancesignal.
 3. A luminance and color separation filter unit (300) as claimedin claim 1, whereby the penalty estimation means (306) is arranged tocompute a first one of the penalty values by means of computing adistance between the first pixel and a second one of the pixels in theneighborhood of the first pixel.
 4. A luminance and color separationfilter unit (300) as claimed in claim 1, whereby the penalty estimationmeans (306) is arranged to compute a first one of the penalty values bymeans of: computing a first difference between a first sub-carrier phaseof the first sample of the composite color television signal,corresponding to the first pixel and a second sub-carrier phase of afirst one of the other samples corresponding to other pixels in theneighborhood of the first pixel; and computing a second differencebetween the first difference and a predetermined value.
 5. A luminanceand color separation filter unit (300) as claimed in claim 4, wherebythe predetermined value corresponds to 180°.
 6. A luminance and colorseparation filter unit (300) as claimed in claim 4, whereby thepredetermined value corresponds to 120° and the decoding means (312) arearranged to determine the at least one final value of the set of valuescomprising the final luminance value and the two color valuescorresponding to the first pixel, on basis of the first sample, theparticular sample and a further one of the other samples correspondingto other pixels in a neighborhood of the first pixel.
 7. A luminance andcolor separation filter unit (300) as claimed in claim 1, whereby theother pixels in the neighborhood of the first pixel are located in awindow which is centered around the first pixel and located in a firstfield to which the first pixel belongs.
 8. A luminance and colorseparation filter unit (300) as claimed in claim 1, whereby a firstportion of the other pixels in the neighborhood of the first pixel arelocated in a first window which is centered around the first pixel andlocated in a first field to which the first pixel belongs and a secondportion of the other pixels in the neighborhood of the first pixel arelocated in a second window which is located in a second field.
 9. Aluminance and color separation filter unit (300) as claimed in claim 8,whereby the second window is centered around a central pixel, the firstpixel and the central pixel having mutually equal coordinates.
 10. Aluminance and color separation filter unit (300) as claimed in claim 8,whereby the second window is centered around a central pixel, thedifference between the coordinates of the first pixel and thecoordinates of the central pixel determined by a motion vector,representing motion between parts of the first and second field.
 11. Animage processing apparatus (700) comprising: receiving means (702) forreceiving a composite color television signal, comprising a chrominancesignal being modulated on a sub-carrier which is located in thehigh-frequency part of the frequency spectrum of a luminance signal; anda luminance and color separation filter unit (706) for extracting theluminance signal and two color signals from the composite colortelevision signal, the filter unit (706) comprising: acquisition means(302) to acquire a first sample of the composite color televisionsignal, corresponding to a first pixel and other samples of thecomposite color television signal, corresponding to other pixels in aneighborhood of the first pixel; correlation estimation means (304) toestimate a first set of correlation values representing correlationsbetween the first sample and the respective other samples, on basis ofan initial separation of an approximation of the luminance signal fromthe composite color television signal; penalty estimation means (306) toestimate a second set of penalty values representing relations betweenthe first sample and the respective other samples; computing means (308)to compute a third set of combined values by means of combiningrespective elements of the first set of correlation values and thesecond set of penalty values; selection means (310) to select aparticular sample of the composite color television signal on basis ofthe corresponding combined value compared to further combined values ofthe third set of combined values; and decoding means (312) to determineat least one final value of a set of values comprising a final luminancevalue and two color values corresponding to the first pixel on basis ofthe first sample and the particular sample.
 12. An image processingapparatus (800) as claimed in claim 11, further comprising a displaydevice (804) for displaying images being represented by the luminancesignal and the two color signals.
 13. A TV comprising the imageprocessing apparatus (800) as claimed in claim 12
 14. A method ofextracting a luminance signal and two color signals from a compositecolor television signal, comprising a chrominance signal being modulatedon a sub carrier which is located in the high-frequency part of thefrequency spectrum of the luminance signal, the method comprising:acquiring a first sample of the composite color television signal,corresponding to a first pixel and other samples of the composite colortelevision signal, corresponding to other pixels in a neighborhood ofthe first pixel; estimating a first set of correlation valuesrepresenting correlations between the first sample and the respectiveother samples, on basis of an initial separation of an approximation ofthe luminance signal from the composite color television signal;estimating a second set of penalty values representing relations betweenthe first sample and the respective other samples; computing a third setof combined values by means of combining respective elements of thefirst set of correlation values and the second set of penalty values;selecting a particular sample of the composite color television signalon basis of the corresponding combined value compared to furthercombined values of the third set of combined values; and determining atleast one final value of a set of values comprising a final luminancevalue and two color values corresponding to the first pixel on basis ofthe first sample and the particular sample.
 15. A computer programproduct to be loaded by a computer arrangement, comprising instructionsto extract a luminance signal and two color signals from a compositecolor television signal, comprising a chrominance signal being modulatedon a sub-carrier which is located in the high-frequency part of thefrequency spectrum of the luminance signal, the computer arrangementcomprising processing means and a memory, the computer program product,after being loaded, providing said processing means with the capabilityto carry out: acquiring a first sample of the composite color televisionsignal, corresponding to a first pixel and other samples of thecomposite color television signal, corresponding to other pixels in aneighborhood of the first pixel; estimating a first set of correlationvalues representing correlations between the first sample and therespective other samples, on basis of an initial separation of anapproximation of the luminance signal from the composite colortelevision signal; estimating a second set of penalty valuesrepresenting relations between the first sample and the respective othersamples; computing a third set of combined values by means of combiningrespective elements of the first set of correlation values and thesecond set of penalty values; selecting a particular sample of thecomposite color television signal on basis of the corresponding combinedvalue compared to further combined values of the third set of combinedvalues; and determining at least one final value of a set of valuescomprising a final luminance value and two color values corresponding tothe first pixel on basis of the first sample and the particular sample.